It is known for a pen to adopt a pushing structure that enables the nib of the pen to be retracted. When such a pen is in use, the nib extends outside the shaft of the pen. When not in use, the nib is retracted back into the shaft of the pen. Such a pushing and retractable design enables convenient and portable use of a pen and hence is very popular. However, when the writing material used in a pen, such as a fluorescent pen or marker, is volatile, even a retractable approach does not prevent the writing material from drying. To solve the problem of volatility, an airtight unit can be added to a retractable nib design to ensure that the nib of a volatile pen not only can stretch out and draw back freely, but also is protected against drying when drawn back into the shaft of the pen.
U.S. Pat. Nos. 6,135,660 and 6,231,257 disclose a pen with an airtight unit for the nib of the pen. The nib airtight unit disclosed in these two patents uses a couple of airtight balls made of elastic and soft materials such as rubber to make up a sealing arrangement to protect the nib of a pen. When not in use, the nib draws back and is isolated in a compartment sealed by both balls. While in use, the nib passes through the two balls. At the same time, the balls change shape and roll until the nib stretches out for writing. Such an airtight unit design has severe shortcomings. For example, the airtight balls can easily become wearied and torn because of the friction that results from pressing and rolling. The airtight balls may thus quickly fail to protect the nib of the pen. In addition, the shape of balls may limit the possible shapes of the nib of a pen.
Therefore, there exists a need for an airtight unit for a writing device such as a pen that effectively encloses a retracted nib, that is durable, and that is able to accommodate a variety of nib shapes.